As is known voice services in mobile networks have strict regulations to allow emergency calls to operate correctly. When it comes to legacy Radio Access Technologies (RATs) such as Global System for Mobile communications (GSM), Code Division Multiple Access 2000 (CDMA2000) and Wideband CDMA (WCDMA) they have been developed over quite some time to fulfill these regulations. However, when it comes to LTE that is now being rollout as a high bandwidth data service network the situation is different. The legacy RATs have native voice support whereas LTE can add voice support, including emergency services, using Voice over Internet Protocol (VoIP) anchored in IP Multimedia Subsystem (IMS). If IMS is not supported, whenever an evolved NodeB (eNodeB) detects that a voice call is being set up, the UE will be ordered to perform a transfer to a voice capable RAT, i.e. 2G or 3G legacy networks are used as fallback in order to get access to CS voice capable radio resources.
The added signaling in the LTE network and the resulting transfer to the CS capable RAN will add time to the CS call setup time. Further, if a problem occurs in the process of signaling and transferring preceding the CS fallback, there is usually not enough time to perform a retry or switch of strategy. It is well known that a delay is perceived as particularly disturbing in relation to voice sessions in general. CS fallback might also cause call setup delay in the magnitude of seconds, or worst case, call setup failure. From a user's perspective, this will be perceived as poor quality of service.
One way to reduce or eliminate this call set up time is to let the LTE capable UE camp on one of the legacy RANs and thereby avoid the extra signaling and transferring of it to the CS capable RAN. However, camping on one of the legacy RANs will make it necessary to transfer the UE to the LTE RAN whenever the user needs to take advantage of the high bandwidth data service provided by LTE.
Thus, there is a need to be able to decide how and when to make such a transfer from the legacy RAN to the LTE RAN and also to be able to decide when to make a transfer back from the LTE RAN to the legacy RAN.